The present invention relates to frequency sweeps and, more particularly, to overlapping frequency sweeps operating in the radio frequency range.
Many communications devices utilize frequency sweeps systems, e.g., demodulators, swept receivers and spectrum analyzers. The output of a frequency sweep system is a monotonic frequency function of time. An overlapping frequency sweep provides such functions which overlap in time. Such sweeps are produced when the time between sweeps is less than the differential delay introduced by the sweep.
Frequency sweeps are often produced by time dispersing a pulse as a function of frequency. The pulse is selected to have frequency components over the desired output range of the sweep. A device for adding time delays as a monotonic function of frequency then processes the signal to produce the sweep.
One such dispersion device is a "reflective array compressor" (RAC). The RAC can be manufactured by etching plural slits in a large quartz crystal. The size and placement of the slits determines the dispersion characteristics of the RAC. For example, a RAC can be designed to provide a linear or other specific sweep.
While RACs perform quickly and reliably, they are difficult to manufacture precisely. A RAC designed to produce a linear frequency sweep will generally have some characteristic non-linearities. The cost of manufacturing each RAC typically prohibits large scale rejection of deviating devices.
Consideration has been given to pre- and post-processing the signal to correct for the imprecisions in a RAC. However, it is quite difficult to precisely tune a pulse, so pre-processing has not proved a satisfactory approach. Post-processing is problematic in an overlapping frequency sweep since, at any given instant, the signal to be processed contains components from plural frequency sweeps.